[arepo24]

What does this bid mean and is it a partnership agreement?
Thank you.

[hrothgar]

arepo24, on 2020-December-18, 11:52, said:

What does this bid mean and is it a partnership agreement?
Thank you.

Without discussion, I would assume that X is for penalty

I think that a minority of people actually play this AS penalty, but given the number of different potential meanings that this might plausibly take, it's best to assume penalty.

[morecharac]

arepo24, on 2020-December-18, 11:52, said:

What does this bid mean and is it a partnership agreement?

Very much a partnership agreement thing, one well worth discussing with partner before playing. Defense vs. 1NT was one of the biggest holes in our system before we agreed on a system and it comes up often enough to be worth learning. I'd say it came up several times a year when we played twice a week; for comparison, transers to minors might come up twice in one year or once in two years.

As mentioned above, some people play it as penalty. I mostly saw that used among older players.

Some of the 1NT defenses use X to indicate a one-suited hand.

We play Cappelletti vs. 1NT and use it to show a 1NT opener of our own, which allows us to Stayman and transfers. I understand that there are other Cappelletti users who use it for penalty. (I'm considering switching to a different system because it's impossible to indicate a long club suit below 3C, and two 1NT hands on a deal seems to happen less often than long clubs.)

No doubt there are other systems that give it some stranger meanings.

Most of the meanings to X over 1NT are Alerts under the current system, though I think that might have changed under the new system that kicks in on 1 January 2021.

[Zelandakh]

1. Declarer refers to the player sitting opposite Dummy during the play whereas this question appears to be about the auction.
2. You mention that your side opened 1NT but do not provide an explanation for that. The meaning for X over 1NT is for many pairs dependant on this explanation.
3. The default meaning for any Double of a NT call, as laid down by Culbertson in the 30s, is penalty.
4. There are an enormous number of defences played to a 1NT opening bid; some of these use a natural (penalty) double and others use a conventional double.
5. Over a Weak NT, the vast majority of pairs play a penalty double, although conventional doubles are not unheard of.
6. Over a Strong NT, it is very common for X to be conventional rather than penalty but absent an agreement, penalty should be the default.
7. After a Double, most pairs cancel the normal Stayman + transfers and play some kind of runout system. Some of these can be quite complex.
8. The default runouts without agreement are that all 2suit calls are weak and natural. Good hands usually start with a Redouble, although highly distributional game-forcing hands might bid 2NT or higher.

[blackshoe]

What a double of 1NT means is most definitely a matter for partnership discussion and agreement and may well depend on what 1NT means, particularly if 1NT is not natural. If my partner doubled 1NT and we hadn't discussed it the meaning of the double would be "partner has made a mistake".

Alerts are not necessarily the same world over. The alert regulations to which morecharac refers are the ACBL's.